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Re: Fwd: I-D ACTION:draft-ietf-fax-esmtp-conneg-03.txt

2002-08-05 12:20:00

[bcc'ed to iesg - on the assumption that they might not want to see the 
resulting replies]

The SMTP Conneg draft has been revised, with the primary change being 
addition of two Appendices.  One discusses use of the option when there are 
multiple recipients.  The other discusses use of the option when there is 

I think the two appendices help slightly.  However they also introduce 
new problems not present in the previous draft.  Also, the new version of 
the document fails to correct a number of problems (some of them serious) 
in the previous version.

In particular,

- it is still overbroad, claiming applicability for general email
  when it really works only for a limited subset of content 
  which has an obvious ordering relation for "quality".   
  perhaps illustrating this, all of the examples are for fax.

- it still permits unauthorized parties to make arbitrary and
  unspecified alterations to mail.  

- worse, the new version permits unauthorized parties to make
  arbitrary assertions of recipient capabilities in the absence
  of explicit information.

- the new version also permits multiple conversions to take place,
  potentially degrading the message considerably more than a
  single conversion.

- it still mandates use of particular reply and status codes in cases
  where they are not always appropriate

- it still has the server reporting status codes for conditions that
  are best detected on the client

- it still allows SMTP servers to make assertions of recipient capabilities
  without actual knowledge of those capabilities.

- I still think overloading of RCPT is a pessimal way to do this
  since the command will often need to be issued twice anyway -
  either because of a mixture of recipient capabilities, or because
  the conversion could not be performed on-the-fly.

recommendations for near-term publication:  

- this feature should be renamed and the document reworded to make it 
  clear that it is fax-specific rather than general purpose.  

- misleading text should be fixed, unworkable recommendations should be

- the document should be published as experimental rather than on the 
  standards track, since it doesn't meet the requirements for proposed 

things that need to be fixed before the document should be considered
as standards-track:

- needs to be examined for a wide range of content

- needs to have a well-defined protocol by which senders can request
  conneg "convert-or-fail" semantics on a per-message, and probably
  per-bodypart, basis.   if they are not requested, such semantics
  must not be applied.

- needs to explicitly specify which conversion algorithms are permissible
  and how new conversion standards may be added, since there can be 
  multiple ways to perform a conversion and some are worse than others.

- needs to state that conneg features are only advertised with the 
  recipient's consent and control

- needs to state exactly which parties are authorized to perform

- needs to prevent multiple conversions.

some details are below.  not everything above can be seen from a close-up
view, so the list of details below should be considered separately from
the overall problems listed above.


     When a data source and a receiver have interactive access to 
     each other, the receiver often informs the source of its 
     capabilities, to permit optimized performance or functionality for 
     the interaction.  Classic telephone-based facsimile is an example, 
     as are voice over IP and ESMTP, among Internet applications.  The
     store-and-forward nature of Internet mail is usually assumed to 
     preclude such capabilities exchanges, although the sender in a 
     store-and-forward scenario could benefit from knowing precise 
     details about the receiver.  In some configurations, direct 
     email-based interactions -- with the originating ESMTP client 
     and the destination ESMTP server able to have direct TCP 
     connect -- are possible, such as over an intranet. In addition 
     an end-to-end exchange can ESMTP for hop-by-hop enforcement.   

nit: s/can/can use/ ???

problem: this proposal doesn't provide any way to "enforce" content
negotiation past a single hop.  there is no requirement that the CONNEG
option be relayed from one hop to another.  even if there were, it would
be problematic - it would require earlier SMTPs to advertise capabilities
of recipients for which they did not have information, and it would 
require (and trust) later SMTPs to perform arbitrary and undefined 
conversions on content to satisfy recipient-specific constraints.

     This document defines an SMTP-based service extension [ESMTP1, 
     ESMTP2] for content negotiation, whereby an SMTP client may 
     request information about content capabilities of the target 
     device or system that is serviced by an SMTP server.  

nit: the capability request is on a per-recipient-mailbox basis rather
than a 'target device or system' basis.  of course being able to request
per-recipient-mailbox capabilities is sufficient to request target
device capabilities when all recipients accessed via a single SMTP server 
have the same capabilities.  but the fact that it's a per-recipient
capability makes the proposal more complex to handle for the sender,
and it would be better to make this clearer from the start.

     server may report the target's content capabilities back to the 
     client. This process can be used to emulate a classic facsimile
     start-of-session capabilities negotiation, as well as being used 
     for other email-based services.  The extension is primarily 
     intended for "direct" SMTP transfers, although relayed scenarios 
     are permitted through a series of SMTP servers and are discussed 
     in Appendix B.

     In examples, "C:" and "S:" indicate lines sent by the
     client and server respectively.
     The key words "MUST", "MUST NOT", "SHOULD", "SHOULD
     NOT", and "MAY" in this document are to be interpreted
     as defined in "Key words for use in RFCs to Indicate
     Requirement Levels" [KEYWORDS].

     (1)  The name of the SMTP service extension is

     (2)  The EHLO keyword value associated with this extension
          is "CONNEG"

     (3)  A parameter using the keyword "CONNEG" is added to the
          RCPT command

     (4)  The server responds with a report of the content
          capabilities of the device or system that embodies the
          target RCPT address.

4.1  Parameter

            The client requires support for the capability. If the 
            target does not support the CONNEG parameter, the target 
            MUST reject the RCPT command with a 504 reply.

            If the target can not support the capability due to a 
            temporary problem, it MUST reject the RCPT command with 
            a 404 reply. 

nit:  It is not clear whether it is desirable for the server to give 
CONNEG errors priority over all other sources of error.   For instance,
if a recipient address is invalid, reporting *that* probably results in
a more useful error message than reporting that the CONNEG feature is
not supported for that particular recipient.  

suggestion:  reporting of errors with the CONNEG=REQUIRED option SHOULD 
only occur if the RCPT command would otherwise have a successful result.  
If the RCPT command would fail for other reasons, the error and status
codes should reflect the nature of that failure.  In most cases this will
result in a more useful error message.   An reasonable exception might 
exist if the server checks several potential error conditions concurrently, 
and it determines that the command would fail the CONNEG=REQUIRED criteria
before it determines that the command would fail due to other reasons.

            The client requests the target to use the capability. If 
            the target can not support the capability at this time, the 
            target MUST process the address and message as if the 
            requested CONNEG capabilities had not been specified.
          If the argument does not exist, the default is "REQUIRED".
          When a capability is REQUIRED by the client but can not 
          currently be supported by the target, an error response will 
          have significant performance impact to overall SMTP 
          processing.  Use of the OPTIONAL parameter will ensure high 
          SMTP performance.

nit: s/ensure high SMTP performance/facilitate better SMTP performance/

even better would be to say that REQUIRED SHOULD only be used when the
sender has specified that the message delivery should fail unless it
is known that the recipient has the necessary capability.

even better would be to omit REQUIRED entirely, since it increases the
complexity of the server implementation and the protocol, for essential
zero benefit to the client.  REQUIRED doesn't remove the need for the
client to detect errors due to content negotiation, since it is always
possible that the server/recipient does not support any way of handling
the content at all, or even if such conversion might be possible, the 
client does not have the capability to perform that specific conversion.  

4.2 Client Action
     If the server issued a 250-CONNEG, as part of its
     EHLO response for the current session, the client MAY
     issue the CONNEG parameter with RCPT.
     If the client issues the CONNEG parameter with
     RCPT, then it MUST honor the capabilities specified
     in the CONNEG RCPT reply, and transform data that is
     sent, so that the server can accept the data. The
     client SHOULD transform the data to the "highest" level
     of capability of the server.

problem: the client cannot be compelled to perform transformations that
are impossible or for which the client lacks the capability.  no client
can reasonably convert video to audio. few clients could convert audio
or fax to text, and then only if the audio or fax contained words.
if taken literally this provision would prevent any client from asserting 

suggestion: rather than saying "MUST honor" say that the client
MUST NOT send forward which is inconsistent with the recipient's
stated capabilities.  the client SHOULD, if capable, perform 
transformations which are likely to preserve the recipient's ability
to use most of the information in the content.  if the client is unable
to perform such transformations, it MUST bounce the message.

(there probably needs to be some interaction with critical content
here - it might be acceptable for a SMTP to discard conntent marked
as non-critical rather than bouncing the message.  but under no
circumstances should the message be delivered to the recipient when
the recipient has stated via CONNEG that he/she cannot receive it)

     If the server rejects the RCPT command with a 404 reply, the 
     client may later reissue the RCPT with the CONNEG parameter
     in a different SMTP session.

nit: presumably "may" should be capitalized here.

     If the server returns an EHLO 250 code without CONNEG 
     capabilities, the client MUST NOT issue a CONNEG parameter 
     with RCPT.

     Methods of using of this option with multiple addressees, for 
     the same content, are discussed in Appendix A.

4.3 Server Action
     If the client specifies "CONNEG=REQUIRED" in the RCPT, 
     but the server does not support the CONNEG parameter, the server
     MUST reject the RCPT command with a 504 reply. 
     If the server supports the CONNEG parameter, but can not return 
     the recepient's capability temporarily, the server MUST reject the 
     RCPT command with a 404 reply. For example, if the server gets 
     the capability information from a directory, but its connection is 
     offline, the server MUST reject the RCPT command with a 404 

     If the client specifies "CONNEG=OPTIONAL" in the RCPT, 
     but the server does not support the CONNEG parameter or can not 
     return the recipient's capability temporarily, the server MUST 
     process the address and message as if the requested CONNEG
     capabilities had not been specified.

     Regardless of the value of the parameter, if the server does 
     support the CONNEG parameter and the address is acceptable, 
     then it MUST issue a 250 reply, followed by the capabilities
     of the server that is specified by  the RCPT address.
     Successful responses to CONNEG RCPT requests will always be
     multiple SMTP lines.  The first line is the normal RCPT response,
     and subsequent lines beginning with the exact string
     "250-CONNEG " and "250 CONNEG " are the CONNEG responses.
     The last line begins with "250 CONNEG ".  

     If the SMTP server supports ENHANCEDSTATUSCODES [RFC1893], the 
     response strings for a success are "250-2.1.5 CONNEG" and 
     "250 2.1.5 CONNEG". 

nit: I think you want these to be "250-2.1.5 CONNEG " and "250 2.1.5 CONNEG ".

nit: it is out of scope for this document to specify the status codes for
a successful RCPT command.

     The response strings for indicating a
     permanent failure are "504-5.3.3 CONNEG" and "504 5.3.3 CONNEG". 
     The response strings for a temporary failure are "404-4.3.3 
     CONNEG" and "404 5.3.3 CONNEG".

problem: this overloads existing status codes for new error conditions.
there needs to be new status codes for "sender insisted on not delivering
the message unless it knew for sure that the recipient could deal with
it and the recipient's capabilities could not be obtained".

     All CONNEG-capable clients and CONNEG-capable servers MUST
     be able to successfully process CONNEG lines that are up to 512
     characters long, as required by RFC2821. If the length of CONNEG
     lines is greater than 512 characters, the server MUST insert
     line breaks and make next CONNEG line. 

problem: you need to explain more about how this is done.  e.g. can
"<CR><LF>250-2.1.5 CONNEG " be inserted in the middle of a token?
are spaces at the end of a reply line significant?

     The contents of the capability listing MUST conform to 
     the specifications in "Content Feature Schema for Internet Fax". 

problem: this restriction seems inappropriate if the content is not a fax.

     Command with "CONNEG":
      "RCPT TO:" ("<Postmaster@" domain ">" / "<Postmaster>"
                   / Forward-Path) (SP "CONNEG =" ("REQUIRED" /
                 "OPTIOANL") CRLF

nit: OPTIONAL is misspelled

nit: "CONNEG =" should be "CONNEG="

nit: "<Postmaster@" domain ">" is redundant with Forward-Path

problem: grammar excludes other RCPT options

suggestion: the RCPT command syntax should not be formally described 
in this document; it is adequately described elsewhere and CONNEG
doesn't change the syntax.

      ( ("250-" CRLF) *("250-CONNEG" capability CRLF)
        ("250 CONNEG" capability CRLF) )/
      ( ("250-2.1.5" CRLF) 
          *("250-2.1.5 CONNEG" capability CRLF)
           ("250 2.1.5 CONNEG" capability CRLF) )/
      ("504" CRLF) /
      ("504 5.3.3" CRLF) /
      ("404" CRLF) /
      ("404 4.3.3" CRLF) /

      capability  =  <<as per [RFC2879]>>

problem: this grammar excludes the possibility of other kinds of 
responses besides CONNEG.

problem: need to specify how line wrapping works.

radical suggestion: 

1. server encodes the recipient's conneg string in quoted-printable, with 
the line length limits and wrapping rules per the MIME spec.

2. server then prefixes each of the resulting quoted-printable lines with 
"250-2.1.5 CONNEG "

(perhaps adding 2.1.5 even if ENHANCEDSTATUSCODES are not supported)

3. client collects all lines from the response that begin with
"250-2.1.5 CONNEG ", removes that prefix from each line, and 
processes the resulting set of lines as a quoted-printable encoded 
file in order to obtain the conneg string for the recipient.


6.1  Success Response

     An example of ESMTP sequence with successful RCPT response

     S: 220 IFAX
     C: EHLO
     S: 250-DSN
     S: 250 CONNEG
     C: MAIL FROM:<May(_at_)ifax2(_dot_)jp>
     S: 250 <May(_at_)ifax2(_dot_)jp> sender ok
     C: RCPT TO:<June(_at_)ifax1(_dot_)jp> CONNEG = REQUIRED

nit: should be "CONNEG=REQUIRED"

     S: 250-<June(_at_)ifax1(_dot_)jp> recipient ok
     S: 250 CONNEG (&(image-file-structure=TIFF-minimal)
     S: (MRC-mode=0)(color=Binary)(|(&(dpi=204)
     S: (dpi-xyratio=[204/98,204/196]) )(&(dpi=200)
     S: (dpi-xyratio=[200/100,1]) )(&(dpi=400)
     S: (dpi-xyratio=1) ) )(|(image-coding=[MH,MR,MMR])
     S: (&(image-coding=JBIG)(image-coding-constraint=JBIG-T85)
     S: (JBIG-stripe-size=128) ) )(paper-size=[letter,A4,B4])
     S: (ua-media=stationery) )

nit: example shows inappropriate line wrapping, without explanation.

     C: DATA
     S: 354 okay, send data
     C: <<RFC 2822 message with MIME Content-Type:TIFF-FX
          (  image-file-structure=TIFF-minimal

nit: example shows invalid content-type

nit: example assumes fax.

     S: 250 message accepted
     C: QUIT
     S: 221 goodbye

6.2  Long Response

     An example of successful RCPT response when the length of 
     capability is greater than 512 characters. 

     S: 250-2.1.5<June(_at_)ifax1(_dot_)jp> recipient ok
     S: 250-2.1.5 CONNEG (&(image-file-structure=TIFF-minimal) ...
     S: 250-2.1.5 CONNEG   .....
     S: 250 2.1.5 CONNEG   (color=Binary)

okay, that helps, but it still doesn't show whether line wrapping
can occur in the middle of a conneg token.

     An example of succssful RCPT response when CONNEG-capable 
     server supports ENHANCEDSTATUSCODES.

     S: 250-2.1.5<June(_at_)ifax1(_dot_)jp> recipient ok
     S: 250 2.1.5 CONNEG (&(image-file-structure=TIFF-minimal)
     S: (MRC-mode=0)(color=Binary)(|(&(dpi=204)
     S: (dpi-xyratio=[204/98,204/196]) )(&(dpi=200)
     S: (dpi-xyratio=[200/100,1]) )(&(dpi=400)
     S: (dpi-xyratio=1) ) )(|(image-coding=[MH,MR,MMR])
     S: (&(image-coding=JBIG)(image-coding-constraint=JBIG-T85)
     S: (JBIG-stripe-size=128) ) )(paper-size=[letter,A4,B4])
     S: (ua-media=stationery) )

nit: example assumes fax.

6.3  Failure Response

     An example of ESMTP sequence with parmanent failure RCPT 

     S: 220 IFAX
     C: EHLO
     S: 250-DSN
     C: MAIL FROM:<May(_at_)ifax2(_dot_)jp>
     S: 250 <May(_at_)ifax2(_dot_)jp> sender ok
     C: RCPT TO:<June(_at_)ifax1(_dot_)jp> CONNEG = REQUIRED

nit: should be "CONNEG=REQUIRED"

     S: 504 <June(_at_)ifax1(_dot_)jp> recipient ok

     C: QUIT
     S: 221 goodbye

6.4  Temporary Failure Response

     An example of an ESMTP sequence with temporary failure RCPT 
     response when the value of parameter is "REQUIRED":

     S: 220 IFAX
     C: EHLO
     S: 250-DSN
     S: 250 CONNEG
     C: MAIL FROM:<May(_at_)ifax2(_dot_)jp>
     S: 250 <May(_at_)ifax2(_dot_)jp> sender ok
     C: RCPT TO:<June(_at_)ifax1(_dot_)jp> CONNEG = REQUIRED

nit: should be "CONNEG=REQUIRED"

     S: 404 <June(_at_)ifax1(_dot_)jp> recipient ok

     C: QUIT
     S: 221 goodbye
     retry according to implementation

6.5  Temporary Failure with Optional handling

     An example of an ESMTP sequence with temporary failure RCPT 
     response when the value of parameter is "OPTIONAL":

     S: 220 IFAX
     C: EHLO
     S: 250-DSN
     S: 250 CONNEG
     C: MAIL FROM:<May(_at_)ifax2(_dot_)jp>
     S: 250 <May(_at_)ifax2(_dot_)jp> sender ok
     C: RCPT TO:<June(_at_)ifax1(_dot_)jp> CONNEG = OPTIONAL

nit: should be "CONNEG=OPTIONAL"

     S: 250 <June(_at_)ifax1(_dot_)jp> recipient ok
     C: DATA
     S: 354 okay, send data
     C: <<RFC 2822 message with MIME Content-Type:TIFF-FX
          Per "A Simple Mode of Facsimile Using Internet Mail"

nit: example assumes fax

     S: 250 message accepted
     C: QUIT
     S: 221 goodbye

     On publicatiom of this document by the RFC Editor, the IANA 
     shall register the Content_Negotiation ESMTP extension defined 
     in section 3.

     This ESMTP option calls for a respondent to disclose
     its capabilities.  Mechanisms for determining the
     requestor's authenticated identity are outside the
     scope of this specification.  It is intended that this
     mechanism permit disclosure of public information;
     hence there is no particular need for security
     However there is nothing to prevent disclosure of
     sensitive information that should receive restricted
     distribution.  It is, therefore, the responsibility of
     the disclosing ESMTP server to determine whether
     additional security measures should be applied to the
     use of this ESMTP option.

     A man-in-the-middle attack might change the capabilities reported 
     for a given recipient. For example: Suppose the sender knows the 
     recipient has the ability to view color documents so they mark 
     some things in red in what is otherwise a black and white 
     document. But someone interferes with the returned capabilities, 
     indicating that the recipient only supports black and white. The 
     document is duly downgraded, with the result that the recipient 
     doesn't see what the sender marked. 
     An indirect exposure can occur when the report of a capability 
     implies use of specific software.  If that software is known to 
     have security weaknesses, the capabilities report effectively 
     advertises the associated opportunity to exploit the security 
     For target SMTP servers that require security mechanisms to be in 
     force at the start of the session, the target SHOULD refrain from 
     including the CONNEG parameter in an EHLO response until the 
     requisite security mechanisms are in force.

     For digitally signed content, the use of this option poses a
     special challenge.  Digitally signing content relies on that
     content to be in a particular form.  Use of this option changes
     that form.  Hence an SMTP client that uses this option on
     digitally signed content MUST be able to recompute the digital
     signature of the content. 

     Graham Klyne provided useful suggestions to an earlier

     [ESMTP1]  Klensin, J., Freed, N., Rose, M., Stefferud,
               E. and D. Crocker, "SMTP Service Extensions",
               RFC 1869, November 1995
     [ESMTP2]  Klensin, J., "Simple Mail Transfer Protocol",
               RFC 2821, April 2001.
     [RFC2879] McIntyre, L. and G. Klyne, "Content Feature
               Schema for Internet Fax", RFC 2531, August

     [RFC2305] Toyoda, K., Ohno, H., Murai, J. and  D. Wing, 
               "A Simple Mode of Facsimile Using Internet Mail", 
               RFC 2305, March 1998.

     Kiyoshi Toyoda
     Matsushita Graphic Communication Systems,Inc
     2-3-8 Shimomeguro, Meguro-Ku
     Tokyo 153  JAPAN
     Dave Crocker
     Brandenburg InternetWorking
     675 Spruce Drive
     Sunnyvale, CA  94086  USA

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     This document and the information contained herein is
     provided on an "AS IS" basis and THE INTERNET SOCIETY



     ESMTP permits specification of multiple recipients for the same
     content.  This option specified here can produce information that
     different recipients have different content capabilities.  How 
     this differential information is used will depend upon the 
     requirements of the sender.  This specification does not mandate 
     particular choices. Three scenarios are possible:

     (1) Single Recipient
     For each recipient, conduct a separate ESMTP session.  This
     ensures that each content transfer can be tailored to the
     capabilities of each recipient.  This also consumes the largest
     amount of bandwidth and the largest number of cross-network SMTP
     protocol exchanges

     (2) Common Content

     For a single ESMTP session, issue RCPT commands that obtain
     content capabilities information for each recipient.  With the
     DATA command, send the best content form that can be processed by
     ALL of the recipients.  Some recipients will receive content that
     is below their best capabilities.  However this approach also 
     consumes the least bandwidth and has the fewest cross-network 
     protocol exchanges

problem: this scenario does not work because the sender has no way to know a 
priori whether it is possible to produce a common form that is acceptable
to all recipients.  so any sender that sends multiple RCPT commands in a
single session MUST be prepared to terminate the session (either via RSET
or QUIT) and retry sending only to those recipients that DO have a common 

suggestion: delete scenario 2, as it is not workable. 
     (3) Partial Batching
     This scenario begins the same as the "Common Content" scenario.
     Content capabilities information is obtained during a single ESMTP
     session, with all of the RCPT commands issued together.  The
     difference for this scenario is that the client SMTP then
     terminates the session with an RSET and begins one or more new,
     separate sessions.  Addressees are processed in batches, according
     to the similarity of their capabilities.  This option balances
     consumption of bandwidth with optimization of content, at the cost 
     of a terminated session. 


     ESMTP is a direct transfer mechanism, using a single TCP 
     connection.  It supports Internet mail store-and-forward through 
     such characteristics as global addressing and ESMTP enforcement of 
     global features.  

nit: I'm not at all sure what this last sentence means.

Use of the ESMTP Conneg option is 
     straightforward when the originating ESMTP client is able to 
     directly connect to the destination ESMTP server.  The nature of 
     this option is to return address-specific information that will 
     affect content transmission.  Hence the use of this option in the 
     presence of the in-direct effects of store-and-forward is not 
     This Appendix discusses some styles of use for the ESMTP Conneg 
     option when messages are being relayed.  The Appendix is intended
     only to provide discussion.  It is neither intended to be 
     exhaustive nor restrictive.  Other scenarios are likely and 
B.1  Relay Server Modes

     The key challenge for use of the ESMTP Conneg option is the 
     requirement the receiving ESMTP server be able to return 
     capabilities on behalf of a target addressee.  This requirement 
     translates into two, basic styles of operation for the receiving 
     ESMTP server:
     (1) Server Knows Addressee Capabilities
     Here the receiving server has the necessary details about an 
     addressee, at the time the sending server issues the RCPT 
     response.  Typically, this information will be obtained through a 
     direct, real-time query mechanism, either to a directory 
     containing addressee information or to a service run directly on 
     behalf of the individual addressee and possibly on the addressee's 
     One form of query to the addressee's system is to stack a 
     cascading sequence of ESMTP sessions and RCPT commands 
     together, all the way to the destination ESMTP server. Although 
     theoretically possible, this attempt to turn a multi-hop scenario 
     into a real-time, pseudo-direct query is not practical.  It 
     will most likely result in response delays for the RCPT command 
     that are not acceptable, particularly over the public Internet.
     (2) Server Ensures Conversion
     Fundamental to the use of a capabilities exchange is the 
     requirement that the receiver of capabilities information be able 
     to convert content into a form that is more capable than the 
     default form, when the receiver indicates that it can support the 
     superior form.  That is, use of this option presumes that the 
     sender is holding a "more capable" form of the content and will 
     map it to a "less capable form" if the receiver does not support 
     the superior form.  

problem: there is inherently no way for the sender to know in advance 
whether it can convert the message in a form that is  suitable for the

problem: for most kinds of content there is not an obvious ordering
which determines "more capable" or "less capable".  fax images containing
text are not inherently more or less capable than text - each has different
benefits and drawbacks.  HTML is not inherently more or less capable than 
PDF.   it depends on how the recipient will use the message.  so the 
assumption by which relaying is justified is not valid.

     Hence another style of relaying configuration is to have a relay 
     SMTP respond to an RCPT capability query by indicating that it 
     supports the most capable form.  The sender will pass the best 
     version it can.  The receiver has then taken responsibility for 
     performing later conversions, as necessary, to the next hop in the 
     sequence.  That is, the receiving ESMTP server inherits the same 
     level of responsibility already being held by the sending ESMTP 

problem: this is unworkable unless the SMTP server already knows the
recipient's capabilities, since without knowing those capabilities
the SMTP server cannot be sure that it can convert the message to
something the recipient can accept.  and if the SMTP server already
knows the recipient's capabilities, there is no need for it to lie 
about them.

problem: this introduces the possibility of multiple conversions.  multiple 
conversions can cause considerable lossage due to interference between the 
conversions - e.g. aliasing when audio or image sampling rates are changed, 
interference caused when artifacts in the the output of one audio codec 
appear in  the input of another.

problem: this provision almost certainly allows SMTPs to make assertions
about recipient capabilities without the permission of the recipient.

B.2  Some End-to-End Scenarios

     This section suggests methods of using ESMTP Conneg for achieving 
     an end-to-end service that uses knowledge of recipient 
     capabilities for modifying the content or the handling of the 
     content.  This section is intended only to explore possible 
     scenarios.  Others are feasible and likely. The choice of scenario 
     to support will depend upon particular service policies chosen for 
     a relay.  This specification provides no constraint or guidance 
     about which policies to choose.

     (1) Basic Tranfer betweeen Organizations
     Again noting that alternative configurations and support 
     environments are permitted, a simple example of combining the two 
     modes of receiving server style can be helpful.  The scenario to 
     consider is sending from one organization's email system to 
     another organization's email system, across the Internet.  
     This scenario presumes two, independent email services, one for 
     each organization.  In this simple example, the user has a local 
     ESMTP server and it talks with the organization's Internet email 
     gateway.  Hence there are four ESMTP servers.  The originating 
     server takes the message from the originating user.  It then 
     relays the message to the originating organization's ESMTP 
     gateway.  This gateway, in turn, relays the message to the 
     destination organization's gateway, which finally delivers it to 
     the recipient's server.
     In this simple example, the originating ESMTP server includes a 
     CONNEG option on the RCPT command 

problem: for the originating server to include CONNEG on outgoing mail
without any per-message authorization from the sender presumes that it 
is never desirable to deliver mail unless the recipient capabilities are 
known.  this is a radical departure from current practice and is probably 
not feasible.  

if the example assumes that the organizations have a mutual agreement to 
support CONNEG for all recipients and the outgoing server only asserts 
CONNEG when sending to recipients at that organization, this needs to be
stated explicitly.

     to the originating 
     organization's Internet gateway and the gateway always responds 
     that it supports the highest capabilities.  It is then given the 
     most capable form of the content.
     The originating gateway then performs a RCPT CONNEG exchange with 
     the destination organization's gateway.  This gateway can operate 
     in the same "Ensure Conversion" mode as the originating gateway, 

problem: no it cannot, at least not without more assumptions than are stated.

     or it can perform a real-time query about the addressee's 
     capabilities.  The former mode defers resolving recipient 
     capabilities until the final step.  The real-time query requires 
     both that the query mechanism be timely, in order to avoid RCPT 
     response delays, and that it be accurate, with correct information 
     about the addressee.
     The final exchange is, of course, "direct".  The recipient's ESMTP 
     server is presumed to have easy access to the necessary 
     capabilities information.

     (2) Integration with Multipart/Alternative
     Basic support for Conneg will typically involve a Conneg client 
     that has a single, high-quality version of content and then maps 
     is "down" to the best quality that can be supported by the 
     responding Conneg server.  That is, the sending system will do a 
     conversion.  In another mode of operation the Conneg client 
     already holds a number of different mappings of content at 
     different levels of quality.  The client will use the Conneg 
     response to choose the content quality that is "closest" to the
      capabilities of the receiver.
     MIME's Multipart/Alternative provides a means for a content 
     originator to send a fixed set of content quality choices.  An 
     ESMTP relay in the sequence can then choose to use Conneg as a 
     means of selecting one of the alternatives to transmit, rather 
     than transmitting the full set.  Such a scenario is particularly 
     useful when the communication path changes from high-bandwidth to 
     (3) Content Staging on Retrieval Server
     Rather than converting content to a lower quality, an ESMTP Conneg 
     client might choose to use a Conneg response for choosing not to 
     send the content directly.  It might, instead, remove the content 
     from the message and store the content on a retrieval server (for 
     access through HTTP, FTP or the like.)  It can then put a citation 
     into the message, which points to the stored content.
     For the retrieval server, access security, as well as life-time 
     management of the content on the retrieval server, should be 
     considered. A full discussion of these considerations is out of 
     the scope of this memo."

     (4) Alternate Addressing
     If an ESMTP Conneg client has access to multiple addresses for the 
     same recipient, it might use Conneg to determine which is most 
     capabable for particular content and send the content to that one.
     This scenario requires a number of infrastucture features for 
     which there currently are no standards.

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